The Genetics of SMA

The Genetics of SMA

SMA is an autosomal recessive disease, caused by a deletion or mutation in the Survival Motor Neuron 1 (SMN1) gene. While most people have two copies of the SMN1 gene (one inherited from their mother and the other from their father), carriers of SMA have one normal copy of SMN1 and one mutated, or defective, copy. Having at least one normal SMN1 copy will allow a person to produce more than enough SMN protein to prevent any symptoms of SMA, therefore carriers do not show any symptoms of the disease.

SMA usually occurs in children of couples who are both carriers of SMA, each parent having one normal and one defective SMN1 copy. Because carriers of SMA have an equal chance of passing on a normal or defective copy of SMN1, there are three possible combinations of parental SMN1 copies that determine if the offspring will be affected by SMA. 

In the diagram above the blue color represents the mutated or defective SMN1 gene and the three possibilities from below are illustrated.   

Though all people with SMA have two mutated copies of SMN1, not all patients are affected at the same severity. Other genes, such as SMN2, are modifiers of the disease and can make the symptoms of SMA less severe.

For a more in depth genetic discription of SMA produced by Families of SMA please click here



SMA Statistics

Quick Facts about SMA:
  • SMA is a genetic recessive disease that kills more babies than ANY other inherited disease.  It is the number one genetic killer of children under the age of two.
  • SMA affects the voluntary muscles that are used for activities such as crawling, sitting, walking, eating, breathing and swallowing.  The mind and spirit are no different from that of a healthy baby, but the body eventually fails.
  • 50% of SMA Type I babies don't live to see their 1st birthday.
  • 90% of SMA Type I babies don't live to see their 2nd birthday.
  • 1 in every 40 people, or nearly 10 million Americans, UNKNOWINGLY carries the gene responsible for SMA.  Few have any known family history.
  • SMA is a pan-ethnic disease and does not discriminate based on race, ethnicity or gender.
  • There is currently no treatment and no cure, but the National Institutes of Health (NIH) and the National Institute of Neurological Disorders and Stroke (NINDS) selected SMA as the disease closet to treatment of more than 600 neurological disorders.
  • Researchers estimate that a viable treatment and/or cure is attainable in as little as 5 years - IF provided adequate resources and funding.
  • Because scientists know so much about SMA, it is considered a "model" disease with direct impact on research into many other diseases potentially benefiting millions of people.
  • The American College of Medical Genetics recommends that SMA carrier testing be made available to ALL couples planning a family, regardless of ethnicity or family history.